Solid state lighting systems including one- or two-dimensional arrays of solid state lighting devices are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. Solid state lighting arrays are also commonly used as backlights for small liquid crystal display (LCD) screens, such as LCD display screens used in portable electronic devices. In addition, there has been increased interest in the use of solid state lighting arrays as backlights for larger displays, such as LCD television displays.
A solid state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs). Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid state light emitting device. Typically, a solid state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region.
In an LCD backlight, it is common to arrange LED devices in a linear array on a metal bar, called a “light bar”, which is arranged within an LCD backlight unit to emit light parallel to the LCD screen. The light is directed toward the LCD screen by a light guide in the LCD backlight unit.
As LED applications, such a backlighting applications, transition to the use of fewer light sources, the individual light emitting device packages are required to emit more light per package. Accordingly, the present trend is toward using larger and larger size die to accommodate higher light output requirements. Larger LED dice are generally driven at higher forward currents to obtain the desired light output.
Present technology uses LED devices that are attached to packages, heatsinks and/or submounts with silicone die attach. As a die attach material, silicone is not ideal. It is a poor thermal conductor, which may limit the reliability and/or performance of LED die and packages at the higher drive currents needed for higher light output.
Die attach metals may have better thermal conductivity than silicone. However, conventional die attach metals may be unsuited for attachment to plastic packages, such as those used in backlighting applications.
A typical die attach metal is a eutectic Au/Sn alloy, with 80% Au and 20% Sn (by weight). The Au/Sn 80/20 alloy has good mechanical strength, reliability, and thermal conductivity, and is recognized as a standard die attach alloy.
A challenge with the AuSn 80/20 alloy die attach is the requirement of a hot reflow to form the bond, typically in the range of about 305° C. Many plastic packages or chip-on-board packages are adversely affected by exposure to elevated temperatures. The packages may fail catastrophically at temperatures in excess of 300° C., or may suffer material degradation, for example, browning and/or yellowing of the package, which reduces the reflectivity and hence the brightness of the package.
Furthermore, as the number of devices in a lighting unit, such as a light bar, decreases, the overall voltage across the bar may also decrease, resulting in a need to drop the line (supply) voltage further to the operating voltage of the low-voltage bar that includes fewer, larger LED dice. This may require a more complicated power supply that has more dissipation loss, resulting in lower overall system efficiency.